PROJECT SUMMARY/ABSTRACT Age is the most important risk factor for cancer and other debilitating or life-threatening conditions, but how biological processes change with aging at the molecular level and how these changes contribute to cellular, tissue, and organismal dysfunction remain largely unknown. Filling these gaps in our knowledge is critical for understanding both cancer and aging. Rare autosomal-recessive childhood disorders characterized by cancer and premature aging, such as Mosaic Variegated Aneuploidy (MVA) syndrome, are increasingly recognized as powerful tools for advancing knowledge about fundamental biological processes at the cancer-aging interface. Most MVA cases are linked to mutations in BUBR1, a mitotic checkpoint gene required for proper chromosome segregation. Our long-term goal is to uncover the full spectrum of BubR1 biological functions, define how deficiencies in BubR1 contribute to the development of cancer and other aging-related diseases, and apply the information thus gained to develop innovative therapeutic strategies that will lead to better health for many. As the next step in the pursuit of this goal, our objective here is to understand mechanistically how BubR1 insufficiencies induce cellular senescence and enhance growth factor receptor (GFR) signaling to drive age- related carcinogenesis, and to assess whether aneuploidy and senescence resulting from BubR1 aberrancies activate immune surveillance mechanisms. Based on our preliminary data, we hypothesize that BubR1 insufficiencies drive cellular senescence through unscheduled activation of Wnt/?-catenin signaling, promote neoplastic growth by disrupting GFR endocytosis, and engage the cGAS-Sting cytosolic DNA sensing pathway as a fail-safe mechanism to eliminate senescent and aneuploid cells via the immune system. We propose to test this hypothesis by pursuing three specific aims. In the first aim, we will determine how BubR1 insufficiency drives senescence-mediated premature aging using pharmacological and genetic approaches to inhibit ?- catenin activity in BubR1 progeroid mice. In the second aim, we will establish whether and how BubR1 insufficiencies impair internalization of cancer-associated GFRs by using MVA patient cell lines and MVA mouse models. In the third aim, we will assess the in vivo relevance of cytoplasmic genomic DNA sensing by cGas-Sting in senescence, aneuploidization, cancer and aging using mouse models for BubR1 insufficiency and chromosomal instability combined with Sting knockout mice. The expected overall impact of this innovative proposal is that it will vertically advance our understanding of how a prominent mitotic checkpoint protein safeguards against two significant human health problems, cancer and aging, and how deficiencies of BubR1 disrupt its multifaceted functions in the cell. Importantly, the results are expected to have a positive impact because they will likely pave the way towards transformative clinical interventions for treating or preventing a broad spectrum of human cancers and a subset of age-related diseases that limit healthy lifespan, in addition to conceptually advancing the fields of mitosis, signaling, cancer, and aging.